As a general rule, metallic surfaces are painted to protect them from the effects of weathering exposure and corrosion. Metallic or plastic surfaces are also painted for decorative purposes. This holds especially for the painting of workpieces in the automotive industry. Two-component polyurethane paints (2K-PUR) have achieved acceptance as standard technology, since they cure at low temperatures and permit flexible adaptation of the paint film to the substrates. Paint films of this kind are also distinguished by satisfactory weathering resistance.
Photochemically (light) curing paints, in particular, promise qualitative improvements. Of these, UV-cured paints are principally used. Typical fields of application of such paint systems are found in the electronics industry, printing, furniture, parquet flooring, and paper industry.
The paints used in this context often contain binders having ethylenically unsaturated groups suited for polymerization (reactive groups). Acrylate or methacrylate groups are often used as the reactive groups. Typically, the hardening reaction started by the photochemical initiators (photoinitiators) of the known paint systems is radical polymerization triggered by UV radiation.
Multilayer automobile coatings are generally composed of a sequence of several functional layers that contain an anticorrosion layer, such as a phosphate coating, primer, a pigment-containing base coat and a final transparent clear coat.
The top paint layers generally include:                a base coat for providing color and setting the color effect (such as metallic); and        a clear coat as a protective layer against weathering, solvents and scratches.        
The mechanical properties of the light-curing or UV-curing paint systems are typically better than those of conventional annealed enamels. It is difficult, if not impossible to completely cure pure UV paints in shaded regions. This is especially true of geometrically complex components where the paint is also applied or deposited in shaded regions, depending on the application method used, such as dipping, spraying or spattering.
Curable clear coats having binders based on poly(meth)acrylic functional compounds, polyisocyanate, light stabilizers and photoinitiators are known from European Patent Application No. EP 247 563 A2, which is incorporated by reference herein. The final hardness of the paint films described therein is achieved by additional cross-linking in which a part of the light-cross-linkable binders react via still free hydroxyl groups with polyisocyanates to form urethanes (polyaddition).
From European Patent Application No. EP 540 884 A1, which is incorporated by reference herein, a two-step method is known for producing a multilayer coating for automobiles. The paint formulation used in a final clear coat contains radically and/or
cationically curing binders. The hardening is started by UV radiation, and the final hardness is only attained in a second method step. In the second method step, annealing is preferably carried out at temperatures of approximately 80 to 160° C. In one of the variations of the method, heat-activable radical initiators are also used.
The non-prepublished German Patent Application No. DE 103 45 903.0, which is incorporated by reference herein, discusses a reactive paint for achieving complete hardening under exposure to air which contains binders bearing ethylenically unsaturated groups and which is thermally curable or curable in accordance with the dual-cure method and by radical polymerization induced by UV light or high-energy radiation. The paint contains a combined initiator system including a thermal initiator and a UV initiator or a thermal initiator system. In addition, as binders, it contains reactive oligomers, such as additive compounds of a pentaerythritol derivative and an isocyanurate derivative. A method is also described for curing the paint by radical polymerization in which the polymerization is initially thermally started under air and is then continued as a thermal or photoinitiated process under air or under a protective gas.
The problem associated with the known paint systems is that the mechanical properties of their paint films, such as hardness, are still less than satisfactory. Conventional paint films exhibit a low scratch resistance, so that they can be easily scratched, for example, unintentionally in automatic car washes or by sharp objects used in wanton acts of vandalism. This is especially true of paint systems having binders which cure by thermally initiated polymerization. These problems are not the least attributable to unsuitable curing methods, which only lead to the production of paint films having inferior mechanical properties.